Method for enhanced oil recovery from carbonate reservoirs

ABSTRACT

Method of using direct current (DC) electrokinetics to enhance oil production from carbonate reservoirs The method comprising the steps of selecting an underground formation comprising an Oil-bearing carbonate reservoir, positioning two or more electrically conductive elements at spaced apart locations in proximity to said formation, at least one of said conductive elements being disposed in or adjacent to a bore hole affording fluid communication between the interior of said bore hole and said formation, passing a controlled amount of electric current along an electrically conductive path through said formation, said electric current being produced by a DC source including a cathode connected to one of said conductive elements and an anode connected to another of said conductive elements, said electrically conductive path comprising at least one of connate formation water and an aqueous electrolyte introduced into said formation, and withdrawing oil from at least one of said bore holes.

BACKGROUND OF THE INVENTION

This invention relates to the use of direct current (DC) electrokinetics to enhance oil production from carbonate reservoirs.

Carbonate formations occur naturally as sediments of carbonate materials, especially calcite (CaCO₃) and dolomite (CaMg(CO₃)₂). They are anionic complexes of (CO₃)²⁻ and divalent metallic cations such as calcium, magnesium, iron, zinc, barium, strontium and copper, along with a few other less common elements. Carbonates form within the basin of deposition by biological, chemical and detrital processes and are largely made up of skeletal remains and other biological constituents that include fecal pellets, lime mud (skeletal) and microbially mediated cements and lime mud. A main difference between carbonates and silicious soils is that in carbonates chemical constituents, including coated grains such as ooids and pisoids, cement and lime mud are common, whereas they are not present in most siliciclastic sediments. Carbonate reservoirs owe their porosity and permeability to processes of deposition, diagenesis or fracturing.

Petroleum reservoirs in carbonate formations are porous, permeable rock bodies that contain significant amounts of hydrocarbons. It has been estimated that as much as 60% of the world's oil reserves are present in carbonate reservoirs. However, a substantial portion of these reserves is considered unrecoverable. Among many factors that have contributed to the low recovery rates experienced in these reservoirs, the oil-wettable nature of carbonate rock is particularly problematic. Wettability is generally referred to as the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids. A published report of an evaluation of carbonate reservoir rock cores obtained from all over the world showed that a vast majority of carbonates are oil-wet. Chilingar and Yen, Energy Sources, 7(1): 21-27 (1992).

Knowledge of the wettability of reservoir rock is important, e.g., for making an informed decision about the use of gas injection or water flooding as an appropriate secondary oil recovery means. A water flooding application to stimulate oil-wet rock would be considerably less efficient than if applied to water-wet rock.

Various attempts have been made to alter the wettability and thereby provide enhanced oil recovery from carbonate reservoirs. One such approach involves chemically-enhanced oil recovery from in which a surfactant is used to modify wettability of the matrix rock to be more water-wet, as described in U.S. Pat. No. 7,581,594. Another technique entails the use of imbibing fluids which have the effect of modifying the concentration of potential determining ions that influence the surface charge of carbonate rock, so as to improve its water-wetting nature. Zhang and Austad, Colloids and Surfactants A: Physicochemical and Engineering Aspects, 279(1-3): 179-87 (2006). See also U.S. Pat. No. 4,491,512.

A number of methodologies have been considered for enhanced recovery of high viscosity or “heavy” oil. Low-frequency alternating current (AC) heating has been evaluated in Canadian heavy oil fields. Electro-magnetic (EM) and radiofrequency (RF) induction have been proposed for near well bore heating to reduce oil viscosity. Down-hole resistive heaters have also been suggested for heating the near well bore reservoir rocks. The research and development affiliates of several major oil companies have investigated various AC, RF and down-hole heaters for enhanced oil recovery. None of these approaches have produced consistent results.

Enhanced oil recovery has been achieved by DC electrical stimulation. See, e.g., U.S. Pat. Nos. 6,877,556, 7,322,409 and 7,325,604, which are commonly owned with the present application. To date, this technique has been shown to be effective in formations composed primarily of either sandstone or unconsolidated sand.

Insofar as is known, the use of DC electrokinetics for hydrocarbon recovery enhancement in a carbonate rock reservoir has not previously been proposed.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides an efficient and effective method of enhancing oil recovery from a carbonate reservoir.

This method comprises selecting an underground formation comprising an oil-bearing carbonate reservoir, positioning two or more electrically conductive elements at spaced apart locations in proximity to the formation, at least one of the conductive elements being disposed in or adjacent to a bore hole affording fluid communication between the bore hole interior and the formation, passing a controlled amount of electric current along an electrically conductive path through the formation and withdrawing oil from at least one of the bore holes. The electric current applied in carrying out this method is produced by a DC source including a cathode connected to one of the conductive elements and an anode connected to another of the conductive elements, and the electrically conductive path comprises at least one of connate formation water and an aqueous electrolyte introduced into the formation.

In another aspect, the present invention provides a method of fracturing an oil-bearing carbonate rock formation by subjecting the formation to long term electrical stress.

The invention described herein is believed to be the first technically feasible method using electrokinetic phenomena to enhance oil recovery from a carbonate reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

The following summary as well as the following description will be better understood when read in conjunction with the accompanying figures in which:

FIG. 1 is a schematic diagram of a DC electrokinetic method for enhancing oil recovery from DC oil-bearing carbonate reservoir in accordance with this invention;

FIG. 2 is a schematic diagram of one embodiment of a DC electrokinetic method for enhancing oil recovery from an oil-bearing carbonate reservoir; and

FIG. 3 is a schematic diagram of another embodiment of a DC electrokinetic method for enhancing oil recovery from an oil-bearing carbonate reservoir.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures in general, and to FIG. 1 specifically, there is shown an underground formation 11 composed primarily of oil-bearing carbonate rock, in which the method of the invention is practiced. A suitable carbonate reservoir may be selected or determined using well established geologic sampling techniques. The oil-wet or water-wet condition of the carbonate rock present in the formation can be assessed according to the previously reported method of Chilingar and Yen, supra. Typically, such oil-bearing formations are found beneath the upper strata of earth, commonly referred to as overburden, at a depth on the order of 1,000 feet or more below the surface. Communication from the surface 12 to the formation 11 is established through one or more bore holes. In FIG. 1, communication from the surface 12 to the formation 11 is established through spaced-apart boreholes 13 and 14. Borehole 13 functions as an oil-producing well, whereas the adjacent hole 14 is a special access passage provided for the transmission of electricity to the formation 11.

The present invention can be practiced using a multiplicity of electrically conductive elements or electrodes in one or more boreholes. The boreholes may be drilled in a variety of vertical, horizontal or angular orientations and configurations. In FIG. 1, two electrodes are disposed into vertically drilled boreholes. A first electrode 15 is lowered through access hole 14 to a location in proximity to formation 11. Preferably, first electrode 15 is lowered through access hole 14 to a medial elevation in formation 11, as shown in FIG. 1. By means of an insulated cable in access hole 14, the relatively positive terminal or anode of a high-voltage DC electric power source 2 is connected to the first electrode 15. The relatively negative terminal or cathode of the power source is connected to a second electrode 16 associated with producing well 13. Between the electrodes, the electrical resistance of connate water 4 present in the underground formation 11 is sufficiently low so that current can flow through the formation between the first and second electrodes 15, 16. Although the resistivity of the oil is substantially higher than that of the overburden, the current preferentially passes directly through the formation 11 because this electrically conductive path is much shorter than any alternative path traveling through the overburden to “ground.”

To create the electric field, a periodic voltage is produced between the electrodes 15, 16. In one embodiment, the periodic voltage is established using pulsed DC power. In another embodiment, the voltage may be a DC-biased signal with a ripple component produced under modulated AC power. See U.S. Pat. No. 6,877,556. The voltage may be produced using any technology known in the electrical art. For example, voltage from an AC power supply may be converted to DC using a diode rectifier. The ripple component may be produced using an RC circuit or through transistor controlled power supplies. Once the voltage is established, the electric current is normally carried by connate formation water. Electrical current is conducted through the formation by naturally occurring electrolytes in the groundwater. If necessary or desirable, aqueous electrolyte may be introduced into the formation to modify the conductivity of the connate water. A detailed system and procedure for injecting electrolyte solution into an underground formation is described in U.S. Pat. No. 3,782,465. See also, U.S. Pat. No. 5,074,986.

The electric potential required for carrying out electrochemical reactions varies for different chemical components in the oil. As a result, the desired intensity or magnitude of the ripple component depends on the composition of the oil and the type of reactions that are desired. The magnitude of the ripple component must reach a potential capable of oxidizing and reducing bonds in the oil compounds. In addition, the ripple component must have a frequency range above about 2 hertz and below the frequency at which polarization is no longer induced in the formation. The waveshape of the ripple may be sinusoidal or trapezoidal and either symmetrical or clipped. Frequency of the AC component is preferably between 50 and 2,000 hertz. However, it is understood in the art that pulsing the voltage and tailoring the wave shape may allow the use of frequencies higher than about 2,000 hertz.

Referring still to FIG. 1, the steps for practicing the method for enhancing oil recovery from a carbonate reservoir will now be described. An electric potential is applied to first electrode 15 so as to raise its voltage with respect to the second electrode 16 and the region of the formation 11 immediately surrounding it. The voltage between the electrodes 15, 16 is preferably no less than 0.4 V per meter of electrode distance. Current flows between the first and second electrodes 15, 16 through the formation 11. Connate water 4 and/or added aqueous electrolyte, as the case may be, in the interstices of the oil-bearing formation provides a conductive path for current flow. Water that collects above the electrodes in the boreholes does not cause a short circuit between the electrodes and surrounding casings. Such short circuiting is prevented because the water columns in the boreholes have relatively small cross sectional areas and, consequently, greater resistances than the formation itself.

As current is conducted across formation 11, electrolysis in the formation water occurs. Electrolysis generates agents that promote oxidation and reduction reactions in the oil. That is, negatively charged interfaces of oil compounds undergo cathodic reduction, and positively charged interfaces of the oil compounds undergo anodic oxidation. These redox reactions tend to cause decomposition of split long-chain hydrocarbons and multi-cyclic ring compounds into lighter-weight compounds, contributing to lower oil viscosity. Redox reactions may be induced in both aliphatic and aromatic oils. As viscosity of the oil is reduced through redox reactions, the mobility or flow of the oil through the surrounding formation is increased so that the oil may be drawn to the recovery well. Continued application of electric current can ultimately produce carbon dioxide through mineralization of the oil. Dissolution of this carbon dioxide in the oil further reduces viscosity and enhances oil recovery.

In addition to enhancing oil flow characteristics, the present invention promotes electrochemical reactions that upgrade the quality of the oil being recovered. Some of the electrical energy supplied to the oil formation liberates hydrogen and other gases from the formation. Hydrogen gas that contacts warm oil under hydrostatic pressure can partially hydrogenate the oil, improving the grade and value of the recovered oil. Oxidation reactions in the oil can also enhance the quality of the oil through oxygenation.

Electrochemical reactions are sufficient to decrease oil viscosities and promote oil recovery in most applications. In some instances, however, additional techniques may be required to adequately reduce retentive forces and promote oil recovery from underground formations. As a result, the foregoing method for enhanced oil recovery may be used in conjunction with other processes, such as electrothermal recovery or electroosmotic treatment. For instance, electroosmotic pressure can be applied to the oil deposit by switching to straight DC voltage and increasing the voltage gradient between the electrodes 15, 16. Supplementing electrochemical stimulation with electroosmosis may be conveniently executed, as the two processes use much of the same equipment. See U.S. Pat. No. 3,782,465.

Many aspects of the foregoing invention are described in greater detail in related patents, including U.S. Pat. No. 3,724,543, U.S. Pat. No. 3,782,465, U.S. Pat. No. 3,915,819, U.S. Pat. No. 4,382,469, U.S. Pat. No. 4,473,114, U.S. Pat. No. 4,495,990, U.S. Pat. No. 5,595,644 and U.S. Pat. No. 5,738,778. Carbonate reservoirs in which the methods described herein can be applied include, without limitation, those containing heavy oil, kerogen, asphaltinic oil, napthalenic oil and other types of naturally occurring hydrocarbons. In addition, the methods described herein can be applied to both homogeneous and non-homogeneous formations.

The above-described method may be used in combination with one or more pre-treatments to improve the permeability of the formation. For example, the present method may be used in conjunction with an acidizing pre-treatment. A suitable acid is introduced into one or more borehole and an electric field is applied, as described above, to drive the acidizing agent into the formation. Migration of the acid is promoted by electroosmosis, but may be assisted by other means, such as well pumping. The electric field is effective to drive the acid into regions of the formation that cannot readily be reached using other available procedures.

The present invention can be practiced using a multiplicity of cathodes and anodes placed in vertical, horizontal or angular orientations and configurations, as stated earlier. Referring now to FIG. 2, an alternate system is shown with electrodes installed in well casing 113, 114. The well casings 113, 114 extend in a generally horizontal orientation through an oil-bearing formation 111. The relatively positive terminal (anode) of high-voltage DC electric power source 102 is connected to the first well casing 113. The relatively negative terminal (cathode) on the power source is connected to the second well casing 114. In this arrangement, well casing 113 acts as a cathode at the producing well, and well casing 114 acts as an anode. Insulating components or breaks 115 are placed in each of the well casings 113, 114 so that electricity flows between the horizontal sections of the casings within the oil-bearing formation 111. Between the well casings 113, 114, the electrical resistance of the connate water in the formation, or any added aqueous electrolyte, as the case may be, is sufficiently low so that current can flow through the formation between the casings. Although the resistivity of the oil is substantially higher than that of the overburden, the current preferentially passes directly through the formation 111 because this path is much shorter than any path through the overburden to “ground.”

The present method may include one or more electrodes placed at ground level. See, e.g., U.S. Pat. No. 4,495,990. Referring now to FIG. 3, an alternate oil recovery system is shown with a first electrode 215 placed below the earth's surface (marked “E”) and a second electrode 216 is located at ground level in proximity to an underground oil-bearing formation 211. The first electrode 215 is disposed in a borehole 214 that penetrates the formation 211. The first electrode 215 is located within the formation, but may be located outside the formation, depending on the desired deployment and range of the electric field. The second electrode 216 is constructed on the earth's surface. By means of an insulated cable in access hole 214, a terminal on high-voltage DC electric power source 202 is connected to the first electrode 215. The opposite terminal on the power source 202 is connected to the second electrode 216. A voltage difference is established between the first and second electrodes 215, 216 to create an electric field across the formation 211. It should be noted that the second electrode 216 may be contained at a shallow depth just beneath the earth's surface to produce an electric field. For example, the second electrode may be installed within fifty feet of the earth's surface to establish an electric field across the formation. Placing the second electrode 216 at a shallow depth below the earth's surface may be desirable where space above ground is limited.

Although not wishing to be bound by a specific theory, it is believed that when oil-bearing carbonate rock is exposed in situ to DC electrokinetic treatment, as described herein, the wettability of the carbonate rock surface is altered. Specifically, the carbonate rock surface is rendered more hydrophilic than before electrokinetic treatment, thereby causing oil to be more easily displaced from the rock surface, e.g., by water flooding.

The technology described herein may also be beneficially applied to induce fracturing of an oil-bearing carbonate rock formation by subjecting the formation to long term electrical stress.

The fracturing method may be carried out by the steps of positioning two or more electrically conductive elements at spaced apart locations in proximity to the formation; and passing a controlled amount of electric current along an electrically conductive path through the formation, with the electric current being produced by a DC source including a cathode connected to one of the conductive elements and an anode connected to another of the conductive elements, and the electrically conductive path comprising at least one of connate formation water and an aqueous electrolyte introduced into said formation.

Fracturing can be achieved by applying electrical stress in the manner described above for a time period of from 1 day to about 12 months, more preferably from about 1 week to about 6 months, and most preferably for at least 2 weeks. The electrical stress may be applied at 2 volts/cm for the duration of the fracturing treatment, or it may be initiated and maintained at 2 volts for a predetermined time and thereafter reduced to a lower value, e.g., 1 volt/cm.

The following examples describe the invention in further detail. These examples are provided for illustrative purposes only, and should in no way be considered as limiting the invention.

In order to show the viability of the use of electrokinetics for the production of oil from carbonate rock two tests were run in the laboratory. One of the tests was conducted on a core taken from a cap rock and the second on a core taken from a producing petroleum oil reservoir.

Both cores were first saturated with formation brine and then water flooded with 39° API Light Crude oil. Normal laboratory practice for the preparation of these cores was used in both these tests.

Test 1 was performed on the Cap rock which had a diameter of 3.6 cm and a length of 5.5 cm. The core was placed in a sample holder which allowed for a voltage gradient to be established across the core and a Direct Current power supply having a variable current control was used during the test. The measured permeability of this rock was 5.53 mD for water and 94.3 mD to oil. The viscosity of the oil used in this experiment was 19.5. cp. The pressure to saturate the core with water was 40.8 psig, while the pressure necessary to saturate the core with oil was 54.4 psig. A voltage gradient of 2 volts/cm was imposed across the sample. Current was applied from 0 to 244 mA. The flow of oil and water was observed at various currents and the test established that the lowest current at which flow could be maintained was 82 mA or at a current density of 8.16 mA/cm. In addition to the flow established in this low permeability core when the core was removed from the sample holder the rock had fractured as a result of the current passage through the core which ultimately would have increased the permeability of the rock.

Test 2 was performed on the carbonate reservoir formation rock core, which had a diameter of 1.8 cm and a length of 5.15 cm. The measured permeability of this core to water was measured at 5156 mD and 4204 mD to oil. The pressure needed to saturate this core was much lower, 0.2 psig for water and 4.6 psig to oil. The same voltage gradient of 2 volts/cm was used in this test with a resultant flow of water and oil being observed with a current of 18 to 21 mA.

The test results described above are summarized in the following table.

Cum L D c.s.Area L/A PV Q visc. Press. K Vol cm cm2 cm2 cm−1 cc ml/min cp Psi md ml Por Vol

5.50 3.60 10.18 0.540 11.46 2 1.04 49.80 5.53 25 2.0

5.15 1.80 2.54 2.024 12.68 2 1.04 0.20 5156.70 50 4.0

5.50 3.60 10.18 0.540 12.68 2 19.504 54.40 94.93 75 5.0

5.15 1.80 2.54 2.024 12.68 2 19.504 4.60 4204.77 100 6.0

indicates data missing or illegible when filed

The results of these tests demonstrate that the use of electrokinetics can be effective to move oil and water under a voltage stress and current flow that will depend on the initial permeability of the formation, the salinity of the formation and the applied current.

A number of patent and non-patent publications are cited in the foregoing specification in order to describe the state of the art to which this invention pertains. The entire disclosure of each of these publications is incorporated by reference herein.

While certain embodiments of the present invention have been described and/or exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is, therefore, not limited to the particular embodiments described and/or exemplified, but is capable of considerable variation and modification without departure from the scope of the appended claims.

Furthermore, the transitional terms “comprising”, “consisting essentially” of and “consisting of”, when used in the appended claims, in original and amended form, define the claim scope with respect to what unrecited additional claim elements or steps, if any, are excluded from the scope of the claim(s). The term “comprising” is intended to be inclusive or open-ended and does not exclude any additional, unrecited element, method, step or material. The term “consisting of” excludes any element, step or material other than those specified in the claim and, in the latter instance, impurities ordinary associated with the specified material(s). The term “consisting essentially of” limits the scope of a claim to the specified elements, steps or material(s) and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. All methods of enhancing oil recovery from carbonate reservoirs that embody the present invention can, in alternate embodiments, be more specifically defined by any of the transitional terms “comprising”, “consisting essentially of” and “consisting of”. 

1. A method of enhancing oil recovery from a carbonate reservoir comprising the steps of: a. selecting an underground formation comprising an oil-bearing carbonate reservoir; b. positioning two or more electrically conductive elements at spaced apart locations in proximity to said formation, at least one of said conductive elements being disposed in or adjacent to a bore hole affording fluid communication between the interior of said bore hole and said formation; c. passing a controlled amount of electric current along an electrically conductive path through said formation, said electric current being produced by a DC source including a cathode connected to one of said conductive elements and an anode connected to another of said conductive elements, said electrically conductive path comprising at least one of connate formation water and an aqueous electrolyte introduced into said formation; and d. withdrawing oil from at least one of said bore holes.
 2. The method of claim 1, wherein the electrically conductive element to which said cathode is connected is disposed in a bore hole from which oil is withdrawn.
 3. The method of claim 1 further including the step of superimposing an AC component on the DC current to effect decomposition of the withdrawn oil and a decrease in the viscosity thereof.
 4. The method of claim 1, wherein said formation undergoes an acidizing pre-treatment to increase permeability of said formation.
 5. A method of fracturing an oil-bearing carbonate rock formation, said method comprising subjecting said formation to long term electrical stress.
 6. The method of claim 5, wherein said electrical stress is applied to said formation by means comprising: a. positioning two or more electrically conductive elements at spaced apart locations in proximity to said formation; and b. passing a controlled amount of electric current along an electrically conductive path through said formation, said electric current being produced by a DC source including a cathode connected to one of said conductive elements and an anode connected to another of said conductive elements, said electrically conductive path comprising at least one of connate formation water and an aqueous electrolyte introduced into said formation.
 7. The method of claim 6, wherein said electrical stress is applied for a time period ranging from 1 day to 12 months. 